Worst/Best case it's a bug that's happening only for me. Just a second.

If it is an error happening just for you it could be related to either an older version of the campaign (if you haven't updated) or alternatively another add-on that you have installed? Do you think either of these could be the issue?

It's updated and all my other add-ons are campaigns and their dependencies. And there are a lot of them. ._.

Okay well I will just assume its unrelated to the other add-ons because that would be very strange to get a string error. I will have a look online to see if anyone else has this issue. Thank you for reporting it hopefully I will be able to get to the bottom of this.

Can you go into your add-ons folder (under windows Documents/My Games/Wesnoth 1.14/Data/add-ons/ ) and move everything out of it into a folder on the desktop, except the Reign of the Lords, Rulers of Irdya add-ons.

Then with just those 2 add-ons can you try and launch the campaign. (note that it won't break any of the other add-ons you can just drag them back when you have finshed)

This new release adds a bunch of new units but the main change is the end of the old Reign of the Lords default unit advancement add-on.

The other add-on is now officially dead and gone (which should have happened a long time ago). Now all you need to do to turn no advancements is go into the campaign menu and activate the Reign of the Lords modification.

It is way easier and less complicated and thanks to gfgtdf works really well! Also this new addition avoid corruption or conflicting add-ons as we have had problems with in the past.

Also note that 1.12 is no longer receiving updates for this add-on anymore. The 1.14 version of this add-on has now reached 6700 downloads and doubled the 1.12 version in download count !!